Java HashSet class with Examples, Usage

Java Collection API HashSet Class

In Java, HashSet is defined as class and introduced in version 1.2 as part of Collection api. It is available in package java.util.HashSet.

HashSet is implemented based on Set interface to not allow duplicate values.

HashSet API Code

public class HashSet extends AbstractSet implements , Cloneable, Serializable

This is a common collection class and an interview question for 2 years of experience.

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.

Best way to make synchronized at creation time to avoid accidental unsynchronized access.

Set s = Collections.synchronizedSet(new HashSet(...));

Thumb rules about HashSet:

 

1) HashSet implements Set interface.
2) HashSet does not maintain the insertion order, elements are returned in random order.
3) HashSet does not allow duplicates, If element is already present then skips adding to the set.
4) By defalut, HashSet is not synchronized.
5) It is fail-fast. Menas while iterating it, if we try to modify the original HashSet instance then would throw ConcurrentModificationException (This is run time exception)
6) HashSet allow null values adding to it and adding multiple nulls results to override older null values. Hence, we will see only one null value in result (allows one null value).
7) Always null is added at the beginning of HashSet.
8) Can be added any type of Object to the HashSet. Eg. String, Integer, User defined objects (Student, JavaW3schools, W3Schools) etc.

HashSet Methods:

 

boolean  add(Element e): Adds the element to this set if it is not already present.
void clear(): Removes all of the elements.
boolean  contains(Object o): Returns true if this set contains the specified element.
boolean  isEmpty(): Returns true if this set contains no elements.
Iterator<E>  iterator(): Returns an iterator over the elements in this set.
boolean  remove(Object o): Removes the specified element from this set if it is present.
int size() : Returns the number of elements in this set (its cardinality).

Tutorial on Java ArrayList

Java Hashset Example Program:

package com.java.w3schools.hashset;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

// Java - W3schools
public class HashSetExample
{
    public static void main(String[] args) 
    {
        // Hashset Declaratin and creating object.
        Set<String> hashSet = new HashSet<String>();
       
        // Adding elements to it
        hashSet.add("White");
        hashSet.add("Pink");
        hashSet.add("Blue");
        hashSet.add(null);
        hashSet.add("Green");
        hashSet.add("Yellow");
       
        // Adding duplicates
        hashSet.add("White");
        hashSet.add("White");
        hashSet.add("White");
        hashSet.add(null);
        hashSet.add("Yellow");
       
        // Iterating HashSet to print its values.
       
        Iterator<String> it = hashSet.iterator();
       
        while(it.hasNext())
        {
            System.out.print(it.next() + " - ");
        }
    }
}

Output:

null - White - Pink - Blue - Yellow - Green -

Please observe the output, we see that output is not in the insertion order and not printed duplicate values.

This is about HashSet in java. Please leave your questions in comments section.